Depends on the terms of the contract. Generally a contract that young is with a team on a year to year basis, and the players are not necesarilly getting paid till then. And until they really get paid they are not usually bound to that particular team. But some players that teams fall in love with, like Rooney, Messi, or even Adu, were tied up from early on by their team. Soccer is just crazy! So much money gets tossed around for players.